crypto: sm2 - Provide sm2_compute_z_digest when sm2 is disabled

When sm2 is disabled we need to provide an implementation of
sm2_compute_z_digest.

Fixes: e5221fa6 ("KEYS: asymmetric: Move sm2 code into x509_public_key")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306231917.utO12sx8-lkp@intel.com/Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

include/crypto/sm2.h

@@ -13,7 +13,16 @@
 
 struct shash_desc;
 
+#if IS_REACHABLE(CONFIG_CRYPTO_SM2)
 int sm2_compute_z_digest(struct shash_desc *desc,
 			 const void *key, unsigned int keylen, void *dgst);
+#else
+static inline int sm2_compute_z_digest(struct shash_desc *desc,
+				       const void *key, unsigned int keylen,
+				       void *dgst)
+{
+	return -ENOTSUPP;
+}
+#endif
 
 #endif /* _CRYPTO_SM2_H */